1. Field of the Invention
This invention relates to an adjustable hydraulic vibration damper having a piston fastened to a piston rod. The piston divides a work cylinder of the vibration damper into two work chambers, which chambers can be filled with damping fluid. To at least partly control the damping force of the vibration damper, an electromagnetically activated damping valve controls passage of fluid between the chambers. The damping valve can have a coil, a magnetic armature, a valve body connected to the armature and a valve seat. The coil and magnetic armature make up an electro-magnet which has, in its range of operation, an armature guide made of soft magnetic material, which armature guide surrounds the armature.
2. Background Information
Known vibration dampers, such as the vibration damper disclosed in German Patent No. 37 12 477, which corresponds to U.S. Pat. No. 4,850,460, include hydraulic, adjustable vibration dampers in which an electromagnetically activated damping valve controls a flow passage for the damping fluid. The damping valve is equipped with an electromagnet and an armature, or a magnetic armature, connected to a valve body. The electromagnet thereby has a magnetic armature designed as a flat armature. Such a design results in an electromagnet whose attractive force decreases over the stroke according to an E-function, i.e. the electromagnet has a very high holding force, but only a low attractive force.
Other types of vibration dampers, such as those dampers disclosed by German Laid Open Patent Applications No. 34 34 877 and No. 40 16 807, which correspond to U.S. Pat. Nos. 4,650,042 and 5,078,240, respectively, also include hydraulically adjustable vibration dampers. These vibration dampers have a damping valve which is equipped with an electromagnet, whereby the electromagnet is designed as a shunt magnet. With such a shunt magnet, there is a cone opposite the magnetic armature, whereby the magnetic armature itself can be conical in its upper portion as disclosed in German Laid Open Patent Application No. 34 34 877, or cylindrical as disclosed in German Laid Open Patent Application No. 40 16 807. As a result of a magnetic flux exiting laterally between the magnetic armature and the shunt edge, there is a significant increase in the magnetic force in the range of attraction compared to flat armature models as disclosed by German Patent No. 37 12 477. The proportion of the axial magnetic flux decreases correspondingly over the stroke, which is reflected in a reduction of the holding force.
Electromagnets in shunt configurations are typical for proportional magnets, whereby the magnet, depending on the shape of the shunt, achieves a force proportional to the current over the work stroke of approximately 2 mm, which force is practically independent of the distance. Electromagnetically adjustable, hydraulic vibration dampers, in particular damping systems operated at high pressure, must have a high magnetic force. If, as a function of the application, forces which are independent of distance are also required with a short work stroke, the effect of a shunt configuration of the prior art is not sufficient to achieve the typical proportional magnetic action. The effect of the magnetic cross flux in the shunt area is then too small, and it is no longer possible to achieve a constant force over the work stroke.